This project has been inititated within the last month, and at present, there are no definitive conclusions to report. We are currently gathering data to determine the histologic and molecular differences in breast cancer brain metastases as a function of differing fractionation regimens for WBRT in a mouse model. We also hope to determine histologic and molecular differences in breast cancer brain metastases as a function of differing single hemibrain radiation doses using immunohistochemical and proteomic methods. Ultimately, we hope to correlate the finding to a quantitative measure of neurocognitive deficit produced from differing radiation dosing and fractionation schedules for treatment of breast cancer brain metastases.